The cause of the secondary hardening in a quenched high speed steel was studied by measuring dilatation, high temperature hardness and electric resistance, and the following conclusions were obtained : (1) The cause of the decrease in hardness by tempering at the temperature 100～350° is the relief of the internal stresses induced by the structural change during quenching. (2) The secondary hardening is due to the precipitation of a special carbide from the residual austenite, and the martensitization of the residual austenite give the phenomenon a small effect
